Cardiac arrhythmias are disturbances in the normal rhythm of the heartbeat. Cardiac arrhythmias may be corrected by delivering an electric shock to the heart.
Ventricular fibrillation is an arrhythmia characterized by a rapid, irregular heart rhythm that may result in ineffective pumping of blood. Ventricular fibrillation is a frequent cause of cardiac arrest. Defibrillation is a reliable treatment for ventricular fibrillation. It is believed that the success of external defibrillation depends on the passage of an adequate current through the chest rather than on the administration of a preset energy.
Although the mechanism of defibrillation is not completely understood, it is believed that a defibrillating shock creates a voltage gradient though the myocardium and the extracellular space of the heart. This gradient drives current into cardiac cells facilitating defibrillation through alteration of the cellular transmembrane potential. If the voltage gradient created falls below the appropriate threshold, all of the fibrillating eddy currents may not terminate or new currents may be initiated, leading to redevelopment of fibrillation. Thus, it is desirable to deliver a large enough current in the appropriate waveform through a large enough amount of myocardial tissue to disrupt these eddy currents and restore unified depolarization. For example, it is desirable to provide a shock vector containing enough ventricular myocardium to disrupt the numerous eddy currents consuming the heart.
Hypothermia depresses cardiac ion channel conduction, which may contribute to sustained ventricular arrhythmias, including ventricular fibrillation. Although it has been observed that profound hypothermia does not hinder ventricular defibrillation in experimental animals where defibrillation is performed within seconds of ventricular fibrillation, the electrical disturbances provoked or exacerbated by hypothermia may adversely affect the ability to successfully defibrillate a patient.
Atrial fibrillation is an arrhythmia characterized by a rapid, irregular heart rhythm and occurs when the electrical impulses in the atria degenerate from their usual organized rhythm into a rapid chaotic pattern. Delivery of an electrical shock may be used as an emergency procedure when symptoms of very low blood pressure, chest pain, or heart failure caused by rapid, irregular atrial fibrillation are present.
Ventricular tachycardia is an arrhythmia characterized by a rapid heart rhythm that arises in the ventricle. During ventricular tachycardia, the output of the heart is decreased due to the rapid ventricular rate and the lack of properly timed or coordinated atrial contraction. In certain an emergency setting, for example, delivery of an electrical shock is also an effective treatment for ventricular tachycardia.
Cardioversion refers to a synchronized discharge of electrical current that is used to convert abnormal heart rhythms back to a normal rhythm. The electrical current is synchronized with the R or S wave of the QRS complex. It is believed that transient delivery of electrical current causes a depolarization of cardiac cells allowing the sinus node to resume normal pacemaker activity. For example, cardioversion may interrupt a self-perpetuating circuit underlying ventricular tachycardia and, thereby, restore a sinus rhythm.